A New Method of Calculating Intraocular Lens Power After Photorefractive Keratectomy

Challenges of refractive cataract surgery in the era of myopia epidemic: a mini-review

Myopia is the leading cause of visual impairment in the world. With ever-increasing prevalence in these years, it creates an alarming global epidemic. In addition to the difficulty in seeing distant objects, myopia also increases the risk of cataract and advances its onset, greatly affecting the productivity of myopes of working age. Cataract management in myopic eyes, especially highly myopic eyes is originally more complicated than that in normal eyes, whereas the growing population of cataract with myopia, increasing popularity of corneal and lens based refractive surgery, and rising demand for spectacle independence after cataract surgery all further pose unprecedented challenges to ophthalmologists. Previous history of corneal refractive surgery and existence of implantable collamer lens will both affect the accuracy of biometry including measurement of corneal curvature and axial length before cataract surgery, which may result in larger intraocular lens (IOL) power prediction errors and a compromise in the surgical outcome especially in a refractive cataract surgery. A prudent choice of formula for cataract patients with different characteristics is essential in improving this condition. Besides, the characteristics of myopic eyes might affect the long-term stability of IOL, which is important for the maintenance of visual outcomes especially after the implantation of premium IOLs, thus a proper selection of IOL accordingly is crucial. In this mini-review, we provide an overview of the impact of myopia epidemic on treatment for cataract and to discuss new challenges that surgeons may encounter in the foreseeable future when planning refractive cataract surgery for myopic patients.

Bibliometric and visualized analysis of myopic corneal refractive surgery research: from 1979 to 2022

Background

Myopic corneal refractive surgery is one of the most prevalent ophthalmic procedures for correcting ametropia. This study aimed to perform a bibliometric analysis of research in the field of corneal refractive surgery over the past 40 years in order to describe the current international status and to identify most influential factors, while highlighting research hotspots.

Methods

A bibliometric analysis based on the Web of Science Core Collection (WoSCC) was used to analyze the publication trends in research related to myopic corneal refractive surgery. VOSviewer v.1.6.10 was used to construct the knowledge map in order to visualize the publications, distribution of countries, international collaborations, author productivity, source journals, cited references, keywords, and research hotspots in this field.

Results

A total of 4,680 publications on myopic corneal refractive surgery published between 1979 and 2022 were retrieved. The United States has published the most papers, with Emory University contributing to the most citations. The Journal of Cataract and Refractive Surgery published the greatest number of articles, and the top 10 cited references mainly focused on outcomes and wound healing in refractive surgery. Previous research emphasized "radial keratotomy (RK)" and excimer laser-associated operation methods. The keywords containing femtosecond (FS) laser associated with "small incision lenticule extraction (SMILE)" and its "safety" had higher burst strength, indicating a shift of operation methods and coinciding with the global trends in refractive surgery. The document citation network was clustered into five groups: (1) outcomes of refractive surgery: (2) preoperative examinations for refractive surgery were as follows: (3) complications of myopic corneal refractive surgery; (4) corneal wound healing and cytobiology research related to photorefractive laser keratotomy; and (5) biomechanics of myopic corneal refractive surgery.

Conclusion

The bibliometric analysis in this study may provide scholars with valuable to information and help them better understand the global trends in myopic corneal refractive surgery research frontiers. Two stages of rapid development occurred around 1991 and 2013, shortly after the innovation of PRK and SMILE surgical techniques. The most cited articles mainly focused on corneal wound healing, clinical outcomes, ocular aberration, corneal ectasia, and corneal topography, representing the safety of the new techniques.

A No-History Multi-Formula Approach to Improve the IOL Power Calculation after Laser Refractive Surgery: Preliminary Results

This retrospective comparative study proposes a multi-formula approach by comparing no-history IOL power calculation methods after myopic laser-refractive-surgery (LRS). One-hundred-thirty-two eyes of 132 patients who had myopic-LRS and cataract surgery were examined. ALMA, Barrett True-K (TK), Ferrara, Jin, Kim, Latkany and Shammas methods were evaluated in order to back-calculate refractive prediction error (PE). To eliminate any systematic error, constant optimization through zeroing-out the mean error (ME) was performed for each formula. Median absolute error (MedAE) and percentage of eyes within ±0.50 and ±1.00 diopters (D) of PE were analyzed. PEs were plotted with corresponding mean keratometry (K), axial length (AL), and AL/K ratio; then, different ranges were evaluated. With optimized constants through zeroing-out ME (90 eyes), ALMA was better when K ≤ 38.00 D-AL > 28.00 mm and when 38.00 D < K ≤ 40.00 D-26.50 mm < AL ≤ 29.50 mm; Barrett-TK was better when K ≤ 38.00 D-AL ≤ 26.50 mm and when K > 40.00 D-AL ≤ 28.00 mm or AL > 29.50 mm; and both ALMA and Barrett-TK were better in other ranges. (p < 0.05) Without modified constants (132 eyes), ALMA was better when K > 38.00 D-AL ≤ 29.50 mm and when 36.00 < K ≤ 38.00 D-AL ≤ 26.50 mm; Barrett-TK was better when K ≤ 36.00 D and when K ≤ 38.00 D with AL > 29.50 mm; and both ALMA and Barrett-TK were better in other ranges (p < 0.05). A multi-formula approach, according to different ranges of K and AL, could improve refractive outcomes in post-myopic-LRS eyes.

Conditional Process Analysis for Effective Lens Position According to Preoperative Axial Length

PURPOSE

To predict the effective lens position (ELP) using conditional process analysis according to preoperative axial length.

SETTING

Yeouido St. Mary hospital.

DESIGN

A retrospective case series.

METHODS

This study included 621 eyes from 621 patients who underwent conventional cataract surgery at Yeouido St. Mary Hospital. Preoperative axial length (AL), mean corneal power (K), and anterior chamber depth (ACD) were measured by partial coherence interferometry. AL was used as an independent variable for the prediction of ELP, and 621 eyes were classified into four groups according to AL. Using conditional process analysis, we developed 24 structural equation models, with ACD and K acting as mediator, moderator or not included as variables, and investigated the model that best predicted ELP.

RESULTS

When AL was 23.0 mm or shorter, the predictability for ELP was highest when ACD and K acted as moderating variables (R2 = 0.217). When AL was between 23.0 mm and 24.5 mm or longer than 26.0 mm, the predictability was highest when K acted as a mediating variable and ACD acted as a moderating variable (R2 = 0.217 and R2 = 0.401). On the other hand, when AL ranged from 24.5 mm to 26.0 mm, the model with ACD as a mediating variable and K as a moderating variable was the most accurate (R2 = 0.220).

CONCLUSIONS

The optimal structural equation model for ELP prediction in each group varied according to AL. Conditional process analysis can be an alternative to conventional multiple linear regression analysis in ELP prediction.

Development of a New Method for Calculating Intraocular Lens Power after Myopic Laser In Situ Keratomileusis by Combining the Anterior-Posterior Ratio of the Corneal Radius of the Curvature with the Double-K Method

Background: A new method, the Iida–Shimizu–Shoji (ISS) method, is proposed for calculating intraocular lens (IOL) power that combines the anterior–posterior ratio of the corneal radius of the curvature after laser in situ keratomileusis (LASIK) and to compare the predictability of the method with that of other IOL formulas after LASIK. Methods: The estimated corneal power before LASIK (Kpre) in the double-K method was 43.86 D according to the American Society of Cataract and Refractive Surgery calculator, and the K readings of the IOL master were used as the K values after LASIK (Kpost). The factor for correcting the target refractive value (correcting factor [C-factor]) was calculated from the correlation between the anterior–posterior ratio of the corneal radius of the curvature and the refractive error obtained using this method for 30 eyes of 30 patients. Results: Fifty-nine eyes of 59 patients were included. The mean values of the numerical and absolute prediction errors obtained using the ISS method were −0.02 ± 0.45 diopter (D) and 0.35 ± 0.27 D, respectively. The prediction errors using the ISS method were within ±0.25, ±0.50, and ±1.00 D in 49.2%, 76.3%, and 96.6% of the eyes, respectively. The predictability of the ISS method was comparable to or better than some of the other formulas. Conclusions: The ISS method is useful for calculating the IOL power in eyes treated with cataract surgery after LASIK.

Intraocular lens power calculation in eyes with previous corneal refractive surgery

Intraocular lens (IOL) power calculation after corneal refractive surgery (CRS) becomes an expanding challenge for ophthalmologists as more and more cataract surgeries after CRS are required. These patients typically also have high expectations as to visual performance. Conventional IOL power calculation schemes frequently provide inaccurate results in these cases. This review aims to summarize and recommend currently available IOL power calculation methods for eyes with the most common CRS methods: radial keratotomy (RK), photorefractive keratectomy (PRK), laser in situ keratomileusis (LASIK), and small incision lenticule extraction (SMILE). To this end, biometry measuring methods and IOL formulas will be explained and combinations of both are proposed. In synopsis, it is evident that the latest generation of vergence formulas exhibit favorable IOL power prediction accuracy in post-CRS eyes, even though the predictive precision of methods in eyes without CRS is not attained. Ray tracing computation, intraoperative aberrometry, and machine learning–based formulas hold potential to further improve refractive outcomes in post-CRS eyes.

Intraocular Lens Power Calculation in Eyes with Previous Excimer Laser Surgery for Myopia: A Report by the American Academy of Ophthalmology

PURPOSE

To review the literature to evaluate the outcomes of intraocular lens (IOL) power calculation in eyes with a history of myopic LASIK or photorefractive keratectomy (PRK).

METHODS

Literature searches were conducted in the PubMed database in January 2020. Separate searches relevant to cataract surgery outcomes and corneal refractive surgery returned 1169 and 162 relevant citations, respectively, and the full text of 24 was reviewed. Eleven studies met the inclusion criteria for this assessment; all were assigned a level III rating of evidence by the panel methodologist.

RESULTS

When automated keratometry was used with a theoretical formula designed for eyes without previous laser vision correction, the mean prediction error (MPE) was universally positive (hyperopic), the mean absolute errors (MAEs) and median absolute errors (MedAEs) were relatively high (0.72-1.9 diopters [D] and 0.65-1.73 D, respectively), and a low (8%-40%) proportion of eyes were within 0.5 D of target spherical equivalent (SE). Formulas developed specifically for this population requiring both prerefractive surgery keratometry and manifest refraction (i.e., clinical history, corneal bypass, and Feiz-Mannis) produced a proportion of eyes within 0.5 D of target SE between 26% and 44%. Formulas requiring only preoperative keratometry or no history at all had lower MAEs (0.42-0.94 D) and MedAEs (0.30-0.81 D) and higher (30%-68%) proportions within 0.5 D of target SE. Strategies that averaged several methods yielded the lowest reported MedAEs (0.31-0.35 D) and highest (66%-68%) proportions within 0.5 D of target SE. Even after using the best-known methods, refractive outcomes were less accurate in eyes that had previous excimer laser surgery for myopia compared with those that did not have it.

CONCLUSIONS

Calculation methods requiring both prerefractive surgery keratometry and manifest refraction are no longer considered the gold standard. Refractive outcomes of cataract surgery in eyes that had previous excimer laser surgery are less accurate than in eyes that did not. Patients should be advised of this refractive limitation when considering cataract surgery in the setting of previous corneal refractive surgery. Conclusions are limited by the small sample sizes and retrospective nature of nearly all existing literature in this domain.

Intraocular Lens Power Calculation in Eyes After Laser In Situ Keratomileusis or Photorefractive Keratectomy for Myopia

Intraocular power calculation is challenging for patients who have previously undergone corneal refractive surgery. The sources of prediction errors for these eyes are well known; however, the numerous formulas and methods available for calculating intraocular lens power in these cases are eloquent testimony to the absence of a definitive solution. This review discusses some of the available methods for improving the accuracy for predicting the refractive outcome for these patients. It focuses mainly on the methods available on the American Society of Cataract and Refractive Surgery (ASCRS) online calculator and provides some practical guidelines for cataract surgeons who encounter these challenging cases.

Intraocular lens calculations after laser vision correction

PURPOSE OF REVIEW

This article describes different strategies for corneal measurements and/or intraocular lens (IOL) calculations and proposes a systematic approach for IOL selection in patients who have undergone laser corneal refractive surgery.

RECENT FINDINGS

Corneal measurements and IOL calculations cannot be obtained accurately with the standard measuring technologies and formulas in patients with history of laser corneal refractive surgery; therefore a variety of methods and formulas, some of which required prerefractive surgery data, have been proposed to improve the accuracy of measurements and calculations. Formulas that do not rely on prerefractive data seem to be as accurate as those that do; therefore the lack of prerefractive data no longer presents an obstacle for accurate IOL selection in these patients.

SUMMARY

Postrefractive patients undergoing cataract extraction and IOL implantation should have corneal measurements and IOL calculations that take into account and compensate for the limitations in accurate measurements and calculations. IOL selection should also aim to compensate for induced spherical aberration according to the ablation pattern.

Estimation of intraocular lens power calculation after myopic corneal refractive surgery: using corneal height in anterior segment optical coherence tomography

PURPOSE

To investigate the feasibility of estimating effective lens position (ELP) and calculating intraocular lens power using corneal height (CH), as measured using anterior segment optical coherence tomography (AS-OCT), in patients who have undergone corneal refractive surgery.

METHODS

This study included 23 patients (30 eyes) who have undergone myopic corneal refractive surgery and subsequent successful cataract surgery. The CH was measured with AS-OCT, and the measured ELP (ELPm) was calculated. Intraocular lens power, which could achieve actual emmetropia (Preal), was determined with medical records. Estimated ELP (ELPest) was back-calculated using Preal, axial length, and keratometric value through the SRK/T formula. After searching the best-fit regression formula between ELPm and ELPest, converted ELP and intraocular lens power (ELPconv, Pconv) were obtained and then compared to ELPest and Preal, respectively. The proportion of eyes within a defined error was investigated.

RESULTS

Mean CH, ELPest, and ELPm were 3.71 ± 0.23, 7.74 ± 1.09, 5.78 ± 0.26 mm, respectively. The ELPm and ELPest were linearly correlated (ELPest = 1.841 × ELPm - 2.018, p = 0.023, R = 0.410) and ELPconv and Pconv agreed well with ELPest and Preal, respectively. Eyes within ±0.5, ±1.0, ±1.5, and ±2.0 diopters of the calculated Pconv, were 23.3%, 66.6%, 83.3%, and 100.0%, respectively.

CONCLUSIONS

Intraocular lens power calculation using CH measured with AS-OCT shows comparable accuracy to several conventional methods in eyes following corneal refractive surgery.

New Scheimpflug camera device in measuring corneal power changes after myopic laser refractive surgery

PURPOSE

To assess the accuracy of a combined Scheimpflug camera-Placido disk device (Sirius, CSO, Italy) in evaluating corneal power changes after myopic photorefractive keratectomy (PRK).

METHODS

Two hundred and thirty-seven eyes of 237 patients that underwent myopic PRK with a refractive error, measured as spherical equivalent, ranging from -10.75 D to -0.5D (mean -4.63 ± 2.21D), were enrolled in this study. Corneal power evaluation using Sirius were performed before, 1, 3 and 6 months after myopic PRK. Mean simulated keratometry (SimK) and mean pupil power (MPP) were measured. Correlations between changes in corneal power, measured with SimK and MPP, and variations in subjective refraction, calculated at corneal plane, were evaluated using Pearson test at every follow up; differences between preoperative and postoperative data were evaluated with the Student paired t-test.

RESULTS

A good correlation has been detected between the variations in subjective refraction measured at corneal plane 1, 3 and 6 months after myopic PRK and both SimK (R(2) = 0.8463; R(2) = 0.8643; R(2) = 0.7102, respectively) and MPP (R(2) = 0.6622; R(2) = 0.5561; R(2) = 0.5522, respectively) but corneal power changes are statistically undervalued for both parameters (p < 0.001).

CONCLUSIONS

Even if our data should be confirmed in further studies, SimK and MPP provided by this new device do not seem to accurately reflect the changes in corneal power after myopic PRK.

Corneal Anterior Power Calculation for an IOL in Post-PRK Patients

PURPOSE

After corneal refractive surgery, there is an overestimation of the corneal power with the devices routinely used to measure it. Therefore, the objective of this study was to determine whether, in patients who underwent photorefractive keratectomy (PRK), it is possible to predict the earlier preoperative anterior corneal power from the postoperative (PO) posterior corneal power. A comparison is made using a formula published by Saiki for laser in situ keratomileusis patients and a new one calculated specifically from PRK patients.

METHODS

The Saiki formula was tested in 98 eyes of 98 patients (47 women) who underwent PRK for myopia or myopic astigmatism. Moreover, anterior and posterior mean keratometry (Km) values from a Scheimpflug camera were measured to obtain a specific regression formula.

RESULTS

The mean (±SD) preoperative Km was 43.50 (±1.39) diopters (D) (range, 39.25 to 47.05 D). The mean (±SD) Km value calculated with the Saiki formula using the 6 months PO posterior Km was 42.94 (±1.19) D (range, 40.34 to 45.98 D) with a statistically significant difference (p < 0.001). Six months after PRK in our patients, the posterior Km was correlated with the anterior preoperative one by the following regression formula: y = -4.9707x + 12.457 (R² = 0.7656), where x is PO posterior Km and y is preoperative anterior Km, similar to the one calculated by Saiki.

CONCLUSIONS

Care should be taken in using the Saiki formula to calculate the preoperative Km in patients who underwent PRK.

IOL power calculation after corneal refractive surgery

PURPOSE

To describe the different formulas that try to overcome the problem of calculating the intraocular lens (IOL) power in patients that underwent corneal refractive surgery (CRS).

METHODS

A Pubmed literature search review of all published articles, on keyword associated with IOL power calculation and corneal refractive surgery, as well as the reference lists of retrieved articles, was performed.

RESULTS

A total of 33 peer reviewed articles dealing with methods that try to overcome the problem of calculating the IOL power in patients that underwent CRS were found. According to the information needed to try to overcome this problem, the methods were divided in two main categories: 18 methods were based on the knowledge of the patient clinical history and 15 methods that do not require such knowledge. The first group was further divided into five subgroups based on the parameters needed to make such calculation.

CONCLUSION

In the light of our findings, to avoid postoperative nasty surprises, we suggest using only those methods that have shown good results in a large number of patients, possibly by averaging the results obtained with these methods.

The accuracy of the double-K adjustment for third-generation intraocular lens calculation formulas in previous keratorefractive surgery eyes

PURPOSE

To evaluate the effect of the double-K (DK) modification on third-generation formulas.

METHODS

Thirty-eight previously myopic and 24 previously hyperopic eyes that underwent phacoemulsification with intraocular lens (IOL) insertion after Laser in situ keratomileusis (LASIK) were evaluated. Pre-LASIK refraction and keratometry, post-LASIK topography, axial length (AL), IOL type and power, and 1-month postphacoemulsification refraction were recorded spherical equivalent after phacoemulsification (SE(postphaco)). Measured corneal power was adjusted using published and validated methods for postmyopic and posthyperopic LASIK. For each eye, and using SE(postphaco), different DK-IOL formulas were used to calculate the corresponding IOL power, the outcome measure, which was compared with the implanted IOL.

RESULTS

DK-Holladay 1 yielded the highest Pearson correlation coefficient (PCC), 0.955 for myopes and 0.943 for high myopes (AL>26 mm). Mean error (ME) and mean absolute error (MAE) for myopes for DK Sanders-Retzlaff-Kraff theoretical formula [DK-SRK/T] were 0.44±0.84 D and 0.75±0.61 D for DK-SRK/T compared with -0.04±0.67 D and 0.52±0.40 D for DK-Holladay 1 (P<0.001 and P=0.016, respectively), and 0.03±0.88 and 0.64±0.58 for DK-Hoffer Q. For high myopes, ME and MAE were 0.75±0.81 D and 0.84±0.69 D for DK-SRK/T, and -0.05±0.74 D (P<0.0001) and 0.57±0.45 D (P=0.019) for DK-Holladay 1. About 29% of DK-SRK/T eyes with large AL had MAE>1.5 D, compared with 0% for DK-Holladay 1 and 14% for DK-Hoffer-Q. Eyes with previous hyperopic LASIK faired similarly for all formulas, with similar PCCs, and only 8% in each category with MAE>1.5 D.

CONCLUSIONS

DK-SRK/T overestimates IOL power in eyes with large AL, especially with concomitant steep pre-lasik keratometry. Among third-generation formulas, DK-Holladay 1 seems more accurate to use in postmyopic LASIK eyes.

New factor to improve reliability of the clinical history method for intraocular lens power calculation after refractive surgery

PURPOSE

To determine whether the refractive error in an eye developing cataract after refractive surgery represents actual regression or is cataract related and whether the method to gather this information would allow the use of history-related formulas in intraocular lens (IOL) power calculation after refractive surgery.

SETTING

Second University of Naples, Naples, Italy.

DESIGN

Case series.

METHODS

The refractive effects, axial length (AL), and mean keratotomy (K) values were evaluated in eyes before and 6 months after photorefractive keratectomy for myopia or for myopic or mixed astigmatism.

RESULTS

The study evaluated 257 eyes of 166 patients (93 women). Before surgery, there was a high correlation between refractive error and the product of AL and K (AL × K) (r(2) = 0.8213). In patients with refractive results close to emmetropia, the mean AL × K was 1005.91 ± 25.88 (SD), meaning that in the range of 954 and 1058, there was a 95% possibility that the patients were almost fully corrected. The following regression formula was obtained to calculate the amount of refractive error independent of cataract onset: Refractive error = -0.0157 × (AL × K) + 16.437.

CONCLUSIONS

The regression formula determined whether the refraction depended on the onset of cataract and estimated the amount of undercorrection or overcorrection that occurred after refractive surgery, leading to improved estimation of the power of the IOL to be implanted. It may allow the use of history-related formulas in IOL power calculation for eyes that have had corneal refractive surgery.

Intraocular lens power calculation after myopic excimer laser surgery: clinical comparison of published methods

PURPOSE

To compare results of intraocular lens (IOL) power calculation methods after myopic excimer laser surgery.

SETTING

Private practice.

METHODS

In this prospective study, eyes having phacoemulsification after myopic excimer laser surgery were classified into Group 1 (preoperative corneal power available, refractive change known), Group 2 (preoperative corneal power available, refractive change uncertain), and Group 3 (preoperative corneal power unavailable, refractive change known even if uncertain). The IOL power was calculated using the following methods: clinical history, Awwad, Camellin/Calossi, Diehl, Feiz, Ferrara, Latkany, Masket, Rosa, Savini, Shammas, Seitz/Speicher, and Seitz/Speicher/Savini.

RESULTS

The lowest mean absolute errors (MAEs) in IOL power prediction in Group 1 (n = 12) and Group 2 (n = 11), respectively, were with the methods of Seitz/Speicher/Savini (0.51 diopter [D] +/- 0.44 [SD] and 0.55 +/- 0.50 D), Seitz/Speicher (0.58 +/- 0.47 D and 0.54 +/- 0.45 D), Savini (0.60 +/- 0.44 D and 0.65 +/- 0.63 D), Masket (0.82 +/- 0.49 D and 0.69 +/- 0.51 D), and Shammas (0.77 +/- 0.43 D and 1.11 +/- 0.50 D). In Group 3 (n = 5), the lowest MAEs were with the methods of Masket (0.23 +/- 0.27 D), Savini (0.49 +/- 0.86 D), Seitz/Speicher/Savini (0.68 +/- 0.36 D), Shammas (0.84 +/- 0.98 D), and Camellin/Calossi (0.91 +/- 0.84 D).

CONCLUSIONS

When corneal power is known, the Seitz/Speicher method (with or without Savini adjustment) seems the best solution to obtain an accurate IOL power prediction. Otherwise, the Masket method may be the most reliable option.

Comparison of simulated keratometric changes induced by custom and conventional laser in situ keratomileusis after myopic ablation: retrospective chart review

PURPOSE

To determine the relationship between the achieved refractive change and the change in simulated keratometry (K) after myopic laser situ keratomileusis (LASIK) and compare this relationship between custom and conventional treatments.

SETTING

Department of Ophthalmology, University of California, Davis, Sacramento, California, and John A. Moran Eye Center, Salt Lake City, Utah, USA.

METHODS

The change in simulated K and the refractive change induced by custom myopic LASIK and conventional LASIK were determined. The relationship between the variables was analyzed by regression methods.

RESULTS

Custom treatment was performed in 106 eyes and conventional treatment in 224 eyes. Simple linear regression analysis did not fit the clinical observation when the refractive change was less than 2.00 diopters (D) of myopic correction with both treatments. Under the linear model and nonlinear model, each unit of refractive change yielded a greater change in corneal topographic power with custom treatment than with conventional treatment. With both treatments, the rate of change in simulated K was not constant and was much more variable with lower amounts of correction. The relationship was more constant and linear with larger amounts of refractive correction.

CONCLUSIONS

The relationship between the measured change in simulated K and the induced refractive change better fit a nonlinear relationship with smaller amounts of refractive correction in custom LASIK and conventional LASIK. Under all forms of analysis, custom treatments yielded a greater per-unit change in corneal curvature than conventional treatments, especially for refractive corrections of 4.00 D and higher.

Origins of the keratometer and its evolving role in ophthalmology

The keratometer, or ophthalmometer as it was originally known, had its origins in the attempt to discover the seat of accommodation in the eye. Since that early beginning, it has been re-invented a number of times, with improvements and modifications made in the original principles of its design for new applications that arose as ophthalmology advanced. The cornea is not only responsible for the majority of the refraction in the eye, but is also readily accessible for measurement and modification. The keratometer's ability to measure the cornea has allowed it to play a central role in critical advances in ophthalmic history. This review describes the origins and principles of this instrument, the novel applications that led to the keratometer's continued resurgences over its nearly 250-year history, and the modern devices that have borrowed its basic principles and are beginning to replace it in common clinical practice.

Calculation of intraocular lens power using Orbscan II quantitative area topography after corneal refractive surgery

PURPOSE

To present the prospective application of the Orbscan II central 2-mm total-mean corneal power obtained by quantitative area topography in intraocular lens (IOL) calculation after refractive surgery.

METHODS

Calculated and achieved refraction and the difference between them were studied in 77 eyes of 61 patients with previous radial keratotomy (RK), RK and additional surgeries, myopic LASIK, myopic photorefractive keratectomy (PRK), or hyperopic LASIK who underwent phacoemulsification without complications in 3 eye centers. All IOL calculations used the average from the central 2-mm Orbscan II total-mean power of maps centered on the pupil without the use of previous refractive data. Six IOL styles implanted within the bag were used.

RESULTS

Using the SRK-T formula, the overall calculated refraction was -0.64+/-0.93 diopters (D). The overall achieved spherical equivalent refraction (-0.52+/-0.79 D; range: -3.12 to 1.25 D; 95% confidence interval [CI]: -0.70/-0.34 D) was +/-0.50 D in 53% of eyes, +/-1.00 D in 78% of eyes, and +/-2.00 D in 99% of eyes. The overall difference between the calculated and achieved refraction (0.12+/-0.93 D, P=.27; range: -2.18 to 2.62 D; 95% CI: 0.09/0.33 D) was +/-0.50 D in 39% of eyes, +/-1.00 D in 77% of eyes, and +/-2.00 D in 96% of eyes. This difference was +/-1.00 D in 77% of eyes with RK (P=.70), 82% of eyes with myopic LASIK (P=.34), and 90% of eyes with myopic PRK (P=.96). In eyes with RK followed by LASIK, a trend toward undercorrection was noted (P=.03). In eyes with hyperopic LASIK, a trend toward overcorrection was noted (P=.005).

CONCLUSIONS

In eyes with previous corneal refractive surgery, IOL power calculation can be performed with reasonable accuracy using the Orbscan II central 2-mm total-mean power. This method had better outcomes in eyes with previous RK, myopic LASIK, and myopic PRK than in eyes with hyperopic LASIK or RK with LASIK.

Calculations of Corneal Power After Corneo-Refractive Surgery from Keratometry and Change of Spectacle Refraction: Some Considerations on the “Clinical History Method”

PURPOSE

To derive corneal power after kerato-refractive laser surgery (KRS) to be used for a subsequent intraocular lens (IOL) power calculation.

MODEL

Based on the proportion of curvatures of the corneal front and back surface, the central thickness, and the ablation characteristics, we demonstrate a vergence-based formalism to derive the equivalent and back vertex corneal power before and after KRS from the preoperative measured keratometry. As a second option, we demonstrate in the paper how to derive the respective values from the postoperative (instead of the preoperative) measured keratometry.

EXAMPLE

Initial refraction before/after KRS, -12.0/-2.0 D; corneal thickness, 550/440 microm; front/back surface power 48.20-5.81 D, measured Zeiss keratometry before KRS, 42.5 D. After KRS, we calculate a corneal front surface power of 39.82 D and an equivalent/back vertex power and keratometry of 34.08/34.48/35.11 D (result of the "Clinical History Method" at spectacle/corneal plane 32.50/33.96 D). Calculated corneal power values are around 2-3 D lower than measured Zeiss keratometry (37.0 D), which will lead to an IOL power overestimation of about 3-4 D and subsequent hyperopia.

CONCLUSIONS

This formalism may help to prevent hyperopia after cataract surgery subsequent to refractive surgery for myopia.

Intraocular lens power calculation after automated lamellar keratoplasty for high myopia

PURPOSE

To report intraocular lens (IOL) power calculation in 2 eyes that were highly undercorrected by previous myopic automated lamellar keratoplasty (ALK).

METHODS

A 35-year-old man underwent bilateral myopic ALK, which caused high residual myopia (-9.0 -4.0 x 171 and -9.5 -4.5 x 74). The patient then underwent cataract surgery with IOL implantation for cataract development. The double-K clinical history method was utilized, and satisfactory IOL power prediction results were obtained. Two no-history IOL power calculation methods (Rosa correcting factor method and Ferrara theoretical variable refractive index method), which involved axial length-dependent modification of the keratometer-measured corneal radius, and 1 no-history IOL power calculation method (Shammas' method), which involved axial length-independent modification of the keratometer-measured corneal power, were tested on these 2 eyes.

RESULTS

In both eyes, the double-K SRK-T clinical history method gave small IOL prediction errors (-0.66 and -0.81 D). The Shammas' no-history method gave a slightly higher IOL prediction error in the right eye (-1.67 D) and a small IOL prediction error in the left eye (-0.74 D). Unacceptable IOL power prediction errors would have resulted if Rosa's correcting factor method (-8.07 and -8.35 D) or Ferrara's theoretical variable refractive index method (-17.56 and -18.51 D) had been applied. When we utilized Rosa's method for the IOL power calculation by assuming that the previous ALK had fully corrected the refractive error, the predicted IOL powers were very close to the benchmark IOL powers (IOL power prediction errors: 1.16 and 0.37 D). When we utilized Ferrara's method with the same assumption, the IOL power prediction errors remained high (-6.32 and -7.16 D).

CONCLUSIONS

For patients who have had previous myopic ALK (and whose eyes are highly undercorrected) and who require cataract surgery and for whom the pre-ALK history is available, the double-K method appears to yield excellent predictive results. However, if the pre-ALK history is not available, the Shammas' no-history method appears to yield better results than the Rosa's or the Ferrara's method. High undercorrection by the previous ALK might have been one of the major reasons why Rosa's method resulted in a high IOL prediction error in these 2 eyes. The cause for the marked IOL prediction error by Ferrara's method in this case, however, remains to be determined.

Correlation Between Attempted Correction and Keratometric Refractive Index of the Cornea After Myopic Excimer Laser Surgery

PURPOSE

Given that the standard keratometric refractive index of 1.3375 is no longer valid after excimer laser surgery, we aimed to investigate how this value changes postoperatively and if any correlation to the attempted correction exists.

METHODS

The pre- and postoperative data of 98 patients who underwent either myopic photorefractive keratectomy (PRK) or LASIK were reviewed. Using postoperative videokeratography, the corneal radius (r) was obtained; the corrected corneal power (Pc) was measured by separately calculating the dioptric power of the anterior and posterior corneal surfaces. The postoperative index of refraction (n(post)) was derived from these values using the formula: n(post) = (rPc) +1.

RESULTS

As the amount of refractive change increases, n(post) progressively decreases (P < .0001, r = 0.9581). Linear regression provided the subsequent formula to calculate the postoperative index of refraction: n(post) = 1.338 + 0.0009856 x attempted correction.

CONCLUSIONS

Myopic PRK and LASIK induce a decrease in the keratometric refractive index. This reduction correlates to the amount of attempted correction. When the latter is known, calculating n(post) may enable the measurement of corneal power and thus provide an additional method for calculating intraocular lens power in eyes that have undergone myopic PRK or LASIK.

Calculating Intraocular Lens Geometry by Real Ray Tracing

PURPOSE

An implementation of real ray tracing based on Snell's law is tested by predicting the refraction of pseudophakic eyes and calculating the geometry of intraocular lenses (IOLs).

METHODS

The refraction of 30 pseudophakic eyes was predicted with the measured corneal topography, axial length, and the known IOL geometry and compared to the manifest refraction. Intraocular lens calculation was performed for 30 normal eyes and 12 eyes that had previous refractive surgery for myopia correction and compared to state-of-the-art IOL calculation formulae.

RESULTS

Mean difference between predicted and manifest refraction for a 2.5-mm pupil were sphere 0.11 +/- 0.43 diopters (D), cylinder -0.18 +/- 0.52 D, and axis 5.13 degrees +/- 30.19 degrees. Pearson's correlation coefficient was sphere r = 0.92, P < .01; cylinder r = 0.79, P < .01; and axis r = 0.91, P < .01. Intraocular lens calculation for the normal group showed that the mean absolute error regarding refractive outcome is largest for SRK II (0.49 D); all other formulae including ray tracing result in similar values ranging from 0.36 to 0.40 D. Intraocular lens calculation for the refractive group showed that depending on pupil size (3.5 to 2.5 mm), ray tracing delivers values 0.95 to 1.90 D higher compared to the average of Holladay 1, SRK/T, Haigis, and Hoffer Q formulae.

CONCLUSIONS

It has been shown that ray tracing can compete with state-of-the-art IOL calculation formulae for normal eyes. For eyes with previous refractive surgery, IOL powers obtained by ray tracing are significantly higher than those from the other formulae. Thus, a hyperopic shift may be avoided using ray tracing even without clinical history.

Präoperative Berechnung der Stärke intraokularer Linsen bei Problemaugen

The removal of the opaque crystalline lens in cataract surgery and its replacement by an artificial lens has become the most successful surgical intervention in the history of medicine. Modern intraocular lenses, today's micro-incision approach and high-end measurement and computational techniques provide restoration of good visual acuity in the majority of cases. Patients with problem eyes not allowing standard procedures for intraocular lens power calculation require special attention. Among them are highly ametropic subjects with very short or long eyes or patients, whose corneal structures are different from normal due to preceding refractive surgery. The special problems for measurement and lens power calculations in these eyes are dealt with in detail. Based on hitherto unpublished clinical data, causes and possible solutions for the existing problems are discussed. Generally, the best available measurement techniques should be applied in these cases. With respect to the algorithms used it has to be made sure that no additional errors are introduced by the algorithms themselves. The popular SRK II formula should therefore not be used any more. If errors are minimized this way, gross postoperative refractive surprises should be avoided even in problem eyes.

New frontiers for the perioperative data method for IOL calculation following corneal refractive surgeries

PURPOSE

To evaluate the efficiency of the perioperative data method for intraocular lens (IOL) calculation after correction of myopia and hyperopia with different techniques, including reoperated cases.

METHODS

Thirty-five eyes (26 patients) that developed cataract after corneal refractive procedures were evaluated retrospectively. They were categorized according to initial error of refraction into myopes and hyperopes and according to types of refractive procedures into ablative, incisional, both, or others. Reoperated cases were also considered. Number of refractive procedures was noted. Time interval between the first procedure and cataract extraction was indicated. Perioperative method was used to calculate the K value. SRK/T formula was used to calculate IOL power. Difference between intended and finally achieved manifest refraction was an indicator for efficiency of the calculation.

RESULTS

Postoperatively, 77.2% of cases had manifest refraction +/-1.5 D of intended refraction. There was no difference between myopes and hyperopes in terms of final manifest refraction, best-corrected visual acuity, and difference between intended and finally achieved manifest refraction. Similarly were groups of different types of surgeries. Efficiency of the method decreased with high axial lengths and low IOL powers. Neither the number of refractive surgeries nor time interval between surgeries affected efficiency of the method.

CONCLUSIONS

The perioperative data method is equally effective for myopes and hyperopes. The types, numbers of refractive procedures, as well as the time interval between refractive surgery and cataract extraction do not alter the credibility of the method. In high degrees of myopia, the method gives less accurate results.

Estimation of true corneal power after keratorefractive surgery in eyes requiring cataract surgery: BESSt formula

PURPOSE

To describe a new formula, BESSt, to estimate true corneal power after keratorefractive surgery in eyes requiring cataract surgery.

SETTING

Moorfields Eye Hospital, London, United Kingdom.

METHODS

The BESSt formula, based on the Gaussian optics formula, was developed using data from 143 eyes that had keratorefractive surgery. The formula takes into account anterior and posterior corneal radii and pachymetry (Pentacam, Oculus) and does not require pre-keratorefractive surgery information. A software program was developed (BESSt Corneal Power Calculator), and corneal power was calculated in 13 eyes that had keratorefractive surgery and required cataract surgery.

RESULTS

In the eyes having phacoemulsification, target refractions calculated with the BESSt formula were statistically significantly closer to the postoperative manifest refraction (mean deviation 0.08 diopters [D] +/- 0.62 [SD]) than those calculated with other methods as follows: history technique (-0.07 +/- 1.92 D; P = .05); history technique with double-K adjustment (0.13 +/- 2.39 D; P = .05); Holladay 2 with K-values estimated with the contact lens method (-0.76 +/- 1.36 D; P = .03); Holladay 2 with K-values from Atlas topographer (Humphrey) (-0.55 +/- 0.61 D; P<.01). Using the BESSt formula, 46% of eyes were within +/-0.50 D of the intended refraction and 100% were within +/-1.00 D.

CONCLUSIONS

The BESSt formula was statistically significantly more accurate than the other techniques tested. Thus, it could significantly improve intraocular lens power calculation accuracy after keratorefractive surgery, especially when pre-refractive surgery data are unavailable.

A regression model for correcting intraocular lens power after refractive surgery independent of preoperative data

PURPOSE

To find a method of calculating intraocular lens (IOL) power that may be independent of preoperative data in eyes that have previously undergone myopic laser in situ keratomileusis (LASIK).

METHODS

In 148 eyes of 75 patients, before and 6 months after LASIK, IOL power was calculated with SRK/T formula utilizing the spherical equivalent as the desired target refraction. Assuming that LASIK does not alter the crystalline lens refractive properties, IOL calculation error (CER) was estimated with this formula: CER = [pre-LASIK IOL power]/[post-LASIK IOL power]. Then the authors used postoperative biometry and Orbscan II corneal topography data in multiple regression models to find the best variables to predict the CER. Predicted amount of error which is calculated independent of preoperative data could be used to correct the post-LASIK calculated IOL: [corrected post-LASIK IOL power] = CER x [post-LASIK IOL power].

RESULTS

A regression model with these predictors was found: axial length in millimeters (L), radius of the anterior corneal surface best fitted sphere in millimeters divided by radius of the posterior corneal surface best fitted sphere in millimeters (AntBFS/PostBFS), corneal central 5 millimeters mean power in diopters divided by corneal central 3 millimeters mean power in diopters (mean 5 mm/mean 3 mm), the post-LASIK IOL power, and the post-LASIK simulated K reading. The model R square was 0.88.

CONCLUSIONS

There is correlation between post-LASIK biometry values and IOL power correction factor. This study presents a new model for further investigation.

Intraocular Lens Power Calculation after Myopic Refractive Surgery

OBJECTIVE

To evaluate the reliability of different methods developed to calculate intraocular lens (IOL) power after corneal refractive surgery.

DESIGN

Retrospective observational case series.

PARTICIPANTS

Preoperative and postoperative data of all eyes that underwent myopic excimer laser surgery in a private practice (Centro Salus, Bologna, Italy) between 1999 and 2004 were reviewed.

INTERVENTION

The following methods were analyzed: videokeratography, clinical history, Shammas' refraction-derived and clinically derived methods, Rosa's correcting factor, Ferrara's variable refractive index, separate consideration of anterior and posterior corneal curvature (with and without preoperative data), Feiz-Mannis' formula and nomogram, and Latkany's regression formulas (based on both average and flattest postrefractive surgery keratometry). The Holladay 1 formula was used for eyes with an axial length between 22 and 24.49 mm and the SRK-T for eyes longer than 24.49 mm. Double-K formulas were also evaluated, when applicable. Each IOL power determined with these methods was compared to a benchmark value, calculated using the preoperative axial length and corneal power and aiming for the preoperative spherical equivalent.

MAIN OUTCOME MEASURE

Mean error in IOL power prediction.

RESULTS

Ninety-eight eyes of 98 patients were analyzed. The double-K clinical history method, Feiz-Mannis' formula, double-K method based on separate consideration of anterior and posterior corneal curvature (with and without preoperative data), and both Latkany's regression formulas were the only methods resulting in a mean IOL power not statistically different (P>0.05) from the benchmark used for comparative purposes.

CONCLUSIONS

When prerefractive surgery data are available, IOL power should be calculated using the double-K clinical history method. Alternative choices may be represented by the Feiz-Mannis' formula, Latkany's regression formulas based on average and flattest postrefractive surgery keratometry, and the double-K method based on separate consideration of anterior and posterior corneal curvatures. A variant of the latter can be used to calculate IOL power when prerefractive surgery data are not available. Further prospective studies based on patients undergoing phacoemulsification after refractive surgery are needed to validate the results of this theoretical comparison.

Analysis of Intraocular Lens Power Calculation for Eyes With Previous Myopic LASIK

PURPOSE

To evaluate the effectiveness of a manual keratometry (K) adjusted value for intraocular lens (IOL) power calculation in patients who underwent cataract surgery following previous myopic LASIK.

METHODS

Sixteen eyes of 14 consecutive patients who underwent cataract surgery after previous LASIK were evaluated retrospectively. All IOL powers were calculated using an adjusted K value (K minus 1.0 diopter [D]) with the Binkhorst II formula aiming for -0.75 to -1.00 D final refraction. Additionally, the IOL power for each eye was retrospectively calculated using K, refractive-derived K, and adjusted K with the Binkhorst II, Holladay I, and SRK/T formulas. The final refraction was used as a criterion of accuracy of each approach.

RESULTS

Uncorrected visual acuity > or = 20/40 was achieved in 14 (87.5%) of 16 eyes. The mean postoperative spherical equivalent refraction was -0.41 +/- 0.57 D (range: +0.50 to -2.00 D). Twelve (75%) of 16 eyes were within +/- 0.50 D of emmetropia and 15 (94%) of 16 eyes were within +/- 1.00 D. No eye was > +1.00 D.

CONCLUSIONS

Using an adjusted K with the Binkhorst II formula, aiming for -0.75 to -1.00 D, and with the Holladay I formula, aiming for -0.50 to -1.00 D, measuring K with a regular manual keratometer permits determination of an IOL power after myopic LASIK without the need of preoperative LASIK refractive data.

Intraocular lens power calculation using ray tracing following excimer laser surgery

PURPOSE

To evaluate intraocular lens (IOL) power calculation using ray tracing in patients presenting with cataract after excimer laser surgery.

METHODS

Ten eyes of seven consecutive patients who presented for cataract surgery following excimer laser treatment without any pre-refractive biometry data were enrolled in this prospective clinical study. Preoperatively, IOL power calculation was performed using a ray tracing software called OKULIX. Keratometry data (C-Scan) were imported and axial length (IOLMaster) was entered manually. Accuracy of IOL power calculation was investigated by subtracting attempted and achieved spherical equivalent.

RESULTS

Mean spherical equivalent was -3.51+/-2.77 D (range -10.38 to -0.5 D) preoperatively and -1.01+/-1.08 D (range -2.5 to +0.75 D) postoperatively. Mean error was 0.31+/-0.84 D, mean absolute error was 0.74+/-0.46 D, and IOL calculation errors ranged from -1.39 to +1.47 D. A total of 40% of eyes were within +/-0.5 D, 70% within +/-1.0 D, and 100% within +/-1.5 D. Three eyes with corneal radii over 10 mm showed calculation errors exceeding +/-1.0 D. Mean best-corrected visual acuity increased from 20/60 to 20/30 postoperatively.

CONCLUSIONS

IOL power calculation after excimer laser surgery can be difficult, especially when pre-refractive keratometry values are not available. In these cases, ray tracing combined with corneal topography measurements provides reliable and satisfactory postoperative results. However, it is advisable to be careful when calculating IOL power for eyes with corneal radii exceeding 10 mm because of slightly higher prediction errors.

A New Formula for Intraocular Lens Power Calculation After Refractive Corneal Surgery

PURPOSE

When calculating the power of an intraocular lens (IOL) with conventional methods in eyes that have previously undergone refractive surgery, in most cases the power is inaccurate. To minimize these errors, a new IOL power calculation formula was developed.

METHODS

A theoretical formula empirically adjusted two variables: 1) the corneal power and 2) the anterior chamber depth (ACD). From the average curvature of the entrance pupil area, weighted according to the Stiles-Crawford effect, the corneal power is calculated by using a relative keratometric index that is a function of the actual corneal curvature, type of keratorefractive surgery, and induced refractive change. Anterior chamber depth is a function of the preoperative ACD, lens thickness, axial length, and the ACD constant. We used our formula in 20 eyes that previously underwent refractive surgery (photorefractive keratectomy [n = 6], laser subepithelial keratomileusis [n = 3], laser in situ keratomileusis [n = 6], and radial keratotomy [n = 5]) and compared our results to other formulas.

RESULTS

Mean postoperative spherical equivalent refraction was +0.26 diopters (D) (standard deviation [SD] 0.73, range: -1.25 to +/- 1.58 D) using our formula, +2.76 D (SD 1.03, range: +0.94 to +4.47 D) using the SRK II, +1.44 D (SD 0.97, range: +0.05 to +4.01 D) with Binkhorst, 1.83 D (SD 1.00, range: -0.26 to +4.21 D) with Holladay I, and -2.04 D (SD 2.19, range: -7.29 to +1.62 D) with Rosa's method. With our formula, 60% of absolute refractive prediction errors were within 0.50 D, 80% within 1.00 D, and 93% within 1.50 D.

CONCLUSIONS

In this first series of patients, we obtained encouraging results. With a greater number of cases, all statistical adjustments related to the different types of surgery should be improved.

Reliability of a new correcting factor in calculating intraocular lens power after refractive corneal surgery

PURPOSE

To test the reliability of a corneal radius correcting factor (R factor) in calculating intraocular lens (IOL) power in eyes that developed cataract after refractive surgery and compare it with the clinical history (CHM) and double-K (DKM) methods.

SETTING

Department of Ophthalmology, Second University of Naples, Naples, Italy.

METHODS

Nineteen eyes from the literature that underwent cataract extraction and IOL implantation after refractive surgery were used to compare actual postoperative and expected refractive errors utilizing the R factor, CHM, and DKM. Intraocular lens powers were calculated with 3 formulas: SRK/T, Hoffer Q and Holladay 1. The differences were evaluated with the Wilcoxon test and Spearman correlation.

RESULTS

With the R factor SRK/T and Holladay 1 formulas gave the best results; 16 (84.2%) and 17 (89.5%) eyes were within +/-2 diopters (D) of emmetropia. With CHM, the best results were obtained using the SRK/T and Holladay 1 formulas; with both formulas 12 (63.2%) eyes were within +/-2 D of emmetropia. With DKM, the best results were obtained using SRK/T and Holladay 1 formulas; with both formulas 10 eyes (52.63%) were in the range of +/-2 D from emmetropia.

CONCLUSIONS

The R factor can be used with the SRK/T or Holladay 1 formula because this method seems comparable or superior to DKM and CHM.

Axial Eye Length Evaluation Before and After Myopic Photorefractive Keratectomy

PURPOSE

To test the accuracy of a new device (IOL Master; Carl Zeiss, Jena, Germany) in detecting axial eye length changes after photorefractive keratectomy (PRK).

METHODS

Pre- and postoperative (1, 3, and 6 months) subjective refraction and axial eye length measurements were performed in 184 consecutive eyes that underwent PRK with the Nidek EC5000 excimer laser (Nidek Technologies, Gamagori, Japan) to treat refractive errors from +0.25 to -16.25 diopters (D) (mean: -5.12 +/- 3.01 D).

RESULTS

The axial eye length measurements ranged from 22.51 to 31.32 mm (mean: 25.61 +/- 1.47 mm) before PRK; from 22.39 to 31.10 mm (mean: 25.48 +/- 1.43 mm) 1 month after PRK; from 23.17 to 31.14 mm (mean: 25.61 +/- 1.36 mm) 3 months after PRK; and from 23.36 to 29.68 mm (mean: 25.58 +/- 1.35 mm) 6 months after PRK. Preoperative and 1-month postoperative data showed a statistically significant difference (P<.001), whereas no significant difference was found between 1 and 3 months (P=.0137) or 3 and 6 months (P=.2422).

CONCLUSIONS

The IOL Master showed a decrease in the axial eye length measurement larger than the theoretical ablation depth and the difference increased as the correction became higher.